(fx< 12 13)
=> #t
(fx< 12 12)
=> #f
(fx< 13 12)
=> #f
(fx< 16 (fx+ 13 1))
=> #f
(fx< 16 (fx+ 13 3))
=> #f
(fx< 16 (fx+ 13 13))
=> #t
(fx< (fx+ 13 1) 16)
=> #t
(fx< (fx+ 13 3) 16)
=> #f
(fx< (fx+ 13 13) 16)
=> #f